In optical data recording, an optical source, typically a laser or laser diode, generates an incident write signal in the form of a radiation beam. The beam is applied to an optical medium to record data thereon as optically-detectable marks. To facilitate proper application of the beam to the medium, certain types of media, such as compact disk recordable (CD-R) media, include a preformed spiral tracking structure typically referred to as a groove or pregroove. The spiral groove may be "wobbled" in a radial direction of the disk about an average groove centerline in order to provide rotational velocity and beam position information in a well-known manner. The dimensions and shape of the groove can vary depending upon the medium, but an exemplary groove width is on the order of 0.4 micron in the radial direction, with adjacent grooves in the spiral separated by about 1.6 microns in the radial direction. In CD-R media, the groove corresponds to a data track on which marks are recorded.
The quality of recorded data in many optical recording systems is generally very sensitive to the cross-track position of the recording spot on the medium. In a CD-R system, for example, data quality rapidly deteriorates when the recording spot deviates from the centerline of the disk groove. A commonly-used technique for maintaining a recording spot on-track is referred to as push-pull tracking. This technique involves measuring an interference pattern caused by the interaction of the recording spot with the groove or other suitable tracking structure on the recording medium to generate a push-pull tracking signal. A tracking servo adjusts the position of the recording spot to keep the push-pull signal at a predetermined optimum value generally referred to as a "tracking offset" or an on-track value. The tracking offset is intended to compensate for static errors such as detector misalignment and optical axis tilt. Prior art techniques typically determine the tracking offset during a calibration period before data recording. One common approach is to make a series of trial recordings.
U.S. Pat. No. 5,440,534, assigned to the present assignee and incorporated by reference herein, discloses a technique for determining the tracking offset in an optical recorder by correlating a mark formation effectiveness (MFE) signal generated during one or more trial recordings with a corresponding push-pull tracking signal. This technique recognizes and makes use of the fact that certain characteristics of the reflected write signal vary with cross-track position of the recording spot, thereby providing a number of advantages over other techniques. For example, the tracking offset can be determined in a single recording pass, as compared to separate record and read passes required by two-pass techniques. In addition, the resulting tracking offset is optimized for recording rather than for reading. With this method, "on-track" is defined as the cross-track position with the highest MFE signal.
The mark formation effectiveness signals described in U.S. Pat. No. 5,440,534 are hereafter termed "density-based" methods. These methods are based on a single reflected write signal which is generated from light received in substantially all of the return aperture. The above-referenced commonly-assigned U.S. patent application Ser. No. 08/666,172, U.S. patent application Ser. No. 60/035,109 and U.S. patent application Ser. No. 60/034,193 describe techniques for measuring mark formation effectiveness that are fundamentally different from prior art "density-based" methods. These new techniques, hereafter referred to as "diffraction-based" methods, are based on reflected write signals generated from light received in different zones of the return aperture. A mark formation effectiveness signal is derived by comparing these reflected write signals. Diffraction-based methods can also be combined with the prior art density-based techniques for improved measurement signal-to-noise and more accurate prediction of the resulting written mark quality.
The push-pull tracking method described above is referred to as a "single spot" technique in that the spot that records and reads data also generates the tracking signal. This reduces system cost and complexity when compared with multi-spot tracking techniques. However, push-pull tracking also suffers from a number of problems. For example, the tracking offset can vary with conditions such as media tilt, lens decenter, optical spot aberrations, and groove asymmetry. These conditions can change from disk to disk and from point to point on a given disk. The tracking offset determined during a calibration period may therefore be unable to keep the recording beam sufficiently on-track in the presence of the changing conditions that can arise during actual data recording. Although the push-pull technique is used as an example, it should be understood that other tracking methods, both "single spot" and "multiple spot" are vulnerable to tracking errors from these effects as well.
As is apparent from the above, a need exists for a dynamic tracking technique in which a recording beam is maintained on-track using mark formation effectiveness information generated during actual data recording.
U.S. Pat. No. 4,118,735 (Wilkinson) and 4,322,837 (Mickelson et. al.) describe a dynamic tracking technique in which a read-back beam is continuously moved back and forth or "dithered" in a cross-track direction. This repetitive cross-track motion is correlated with any resulting variation in the intensity of the light reflected by the medium. When there is no correlation between cross-track motion and reflected light variations, the read-back beam is considered to be "on-track". This cross-track position corresponds to a minimum of the reflected light intensity for a "burn dark" medium (burn dark being defined as a medium where written marks are less reflective than unwritten lands). When there is any correlation between cross-track motion and reflected light variation, the relative phase (positive or negative) of the two signals is an indication of which side of "on-track" the read-back beam is located.
U.S. Pat. No. 5,434,834 (Shinoda et. al.) describes a similar method to keep a light beam on-track. It differs from U.S. Pat. Nos. 4,118,735 and 4,322,837 in that repetitive cross track motion is no longer achieved by dithering the illuminating beam. Rather, it is produced by the periodic side-to-side wobble of a pre-groove, such as found in recordable compact disk (CD-R) media. This relative cross-track motion between illuminating beam and recording medium is detected using well known methods such as push-pull techniques described above. As with U.S. Pat. Nos. 4,118,735 and 4,322,837, a tracking signal is derived by correlating this cross-track motion with variations in the reflected light intensity. Also, as with U.S. Pat. Nos. 4,118,735 and 4,322,837, "on-track" is defined as a minimum in reflected light intensity.
Commonly-assigned U.S. Pat. No. 08/584,933 improves upon these prior art methods by using a mark formation effectiveness (MFE) signal instead of the reflected light intensity of the prior art. By correlating repetitive cross-track motion with variations in an MFE signal, commonly-assigned U.S. Pat. No. 5,646,919 effectively performs a continuous, dynamic version of the static tracking offset calibration described in U.S. Pat. No.5,440,534. Commonly-assigned U.S. Pat. No. 5,646,919 recognizes that an MFE signal, which is a measured characteristic of the reflected write signal, is superior to simply detecting reflected light intensity for this measurement. A number of density-based DRDW techniques are described in commonly-assigned U.S. Pat. No. 5,646,919 to generate an MFE signal.
Mark formation effectiveness measurement is preferable to detecting reflected light intensity for a number of reasons. For example, the reflected light intensity depends directly on the specific write pulse pattern being recorded. Such data dependent fluctuations reduce measurement signal-to-noise as compared with an MFE signal, which is a specific measured characteristic of reflected write pulses and is much less dependent on data pattern. Also, the maximum MFE signal may not occur at the same cross-track position as the minimum reflected light intensity. This can result in a different on-track position for the method of commonly-assigned U.S. Pat. No. 5,646,919 as compared with the method of U.S. Pat. Nos. 4,118,735; 4,322,837; and 5,434,834.
With certain recording systems, however, there is relatively little change is total reflected light intensity during mark formation. With these systems, density-based DRDW techniques will produce low signal-to-noise MFE signals. If such an MFE signal is then used in the method described in commonly-assigned U.S. Pat. No. 5,646,919, the resulting dynamic tracking signal will have reduced signal-to-noise and may be more susceptible to error. There is a need, therefore, for a dynamic tracking method which is more effective with these types of recording systems.